The mechanism of fluid transport by epithelial tissues such as kidney tubules, gallbladder and intestine is under investigation by the use of optical microscopy. Most of this work has involved fluorescence microscopy of living cultured renal cells using new methods developed in this laboratory over the last five years. The goal of these studies is the detailed understanding of the mechanism of fluid transport and the elucidation of the critical parameters that determine the rate and composition of the transported fluid.